The vapor pressure of refrigerants, particularly water, decreases as the temperature decreases, so that the operating pressure is small near the end of discontinuous refrigerant operation.
Vapor desiccants also have higher vapor pressures at higher temperatures and thus have a decreasing capacity for water vapor as the temperature of the system increases.
Therefore, as the cooling process proceeds in a conventional desiccant refrigeration system, the available vapor supply pressure falls, the vapor absorption pressure rises and the capacity of the desiccant decreases, so the characteristics of the evaporation and the desiccant/heat sink portions of the system are not ideally matched. This inequality provides the principal limitation on minimizing the size of such a refrigerant system, and contributes to the time required to cool the system to the designed temperature.
If a portion of the released heat is transferred to a heat sink material, the desiccant temperature is lessened, so that the desiccant can absorb more water vapor. There is an optimum choice of ratio of heat sink mass-desiccant mass which for a given combination of materials gives a minimum total mass and volume for a given short time water absorption which can be determined easily.
Three physical characteristics affect the amount of water vapor which a desiccant can absorb in a short period of time.
The first characteristic is the relationship between desiccant water vapor capacity and desiccant temperature. The amount of water which all common desiccants can contain decreases with temperature due to their decreasing capacity for water vapor due to their higher vapor pressure at higher temperatures.
The second characteristic which affects performance of the desiccant is the amount of chemical reaction energy released when the water is bound into the desiccant. Any water vapor absorption process releases the latent heat of vaporization of the water into the desiccant when the molecule is bound into the desiccant structure of molecular motions.
However, most desiccants also react chemically with the water in an exothermic process. Two commercially available desiccants, Drierite (a calcium sulphate desiccant available from W.A. Hammond Drierite Co. Xenia, OH and Multiform molecular sieve 4A, (a synthetic zeolite molecular sieve available from Multiform Desicants, Inc., Buffalo, New York) exhibit an exothermic chemical reaction when reacting with water. In the case of the Multiform desiccant, the reaction heat is 80% of the water vapor latent heat. Therefore, 1.8 calories of heat are released into the desiccant for every calorie of heat absorbed from the cooled material in the refrigerator evaporator.
The final characteristic is the vapor pressure over the desiccant in a conventional single-use desiccant refrigerator apparatus. In such an apparatus, the desiccant is analogous to both the pump and condenser found in the conventional refrigeration cycle. This value, which increases with temperature, is equivalent to the suction pressure in a compressor refrigerant process at the refrigerator compressor inlet.
This increased pressure (reduced suction) reduces the refrigeration efficiency of the apparatus as the single-use, non-cyclical desiccant refrigeration process proceeds. As the temperature of the material being cooled falls, the refrigerant (e.g. evaporating water) vapor pressure decreases, thereby reducing the pressure of the vapor supplied to the desiccant. Simultaneously, the temperature of the desiccant rises, thereby increasing the vapor pressure of the desiccant and water absorption rises so that vapor flow from water to desiccant becomes small. Conventional desiccant refrigerant devices have not breached the issue of modifying the evaporator pressure decrease with decreasing temperature.